Among all the rechargeable battery technologies, lithium-ion batteries (LIBs) offer superior performance, and are suitable for a main power source in portable electronics. LIBs are also the most promising power source for electric vehicles and are projected to be enablers of smart grids based on renewable energy technologies. For many of these applications, energy density and cycle life stand out as two important technical parameters which need significant improvements. For example, in 2010 the U.S. Department of Energy has put forth a goal to create a LIB with double the energy density of current batteries and a cycle life of 5000 cycles with 80% capacity retention for electric vehicles, in comparison, the typical high energy batteries used in portable electronics only have a cycle life of 500-1000 cycles. Increasing energy density in LIBs requires developing electrode materials with higher charge capacity or higher voltage. Improving cycle life involves stabilizing two critical components of battery electrodes: active electrode materials and their interface with electrolyte—so-called “solid-electrolyte interphase” (SEI).
Recently, silicon (Si) has emerged as one of the most promising electrode materials for next-generation high energy LIBs. It offers a suitable low voltage for an anode and a high theoretical specific capacity up to 4,200 mAh/g based on the formation of Li4.4Si alloy, which is 10 times higher than that of conventional carbon anodes (372 mAh/g corresponding to the formation of LiC6). However, Si expands volumetrically by up to 400% upon full lithium insertion (lithiation) to form the Li4.4Si alloy, and the alloy can contract significantly to return Si upon lithium extraction (delithiation), creating two critical challenges associated with silicon-based anode materials: degradation of the mechanical integrity of Si anode materials and SEI stability.
Stress induced by the large volume changes causes cracking and pulverization of Si anode materials, which leads to loss of electrical contact and eventual capacity fading. This was considered to be main reason for rapid capacity loss in early studies of Si anode materials. Recently, there have been successes in addressing stability issues in these materials by designing nanostructured materials, including composites of carbon nanowires, carbon nanotubes, carbon nanoporous films with Si nanoparticle.
US 2012/0064409 A1 discloses a nano graphene-enhanced particulate that graphene sheets and anode active materials such as Si are mutually bonded or agglomerated. Due to their small size and open space surrounding Si nanoparticle, the strain in nanostructures can be easily relaxed without mechanical fracture. Similarly, JP 2013-30462A discloses a negative electrode comprising an alloy-based negative electrode material particle or an alloy-based negative electrode material whisker; and a carbon film including 1 to 50 graphene layers, wherein a surface of the alloy-based negative electrode material particle or the alloy-based negative electrode material whisker is covered with the carbon film.
JP 2012-056833A discloses a carbon nanostructure and metal-supported carbon nanostructure having a new structure used as a negative electrode material or the like of a lithium ion secondary battery. The carbon nanostructure is produced, in which carbon-containing rod-shaped materials and/or carbon-containing sheet-shaped materials are bound three-dimensionally to each other and alveolar-like voids partitioned by graphene multilayer membrane walls are formed in the rod-shaped materials and/or the sheet-shaped materials. The production method of the carbon nanostructure includes the steps of: blowing methyl acetylene gas into a solution containing a metallic salt to form rod-shaped crystalline materials and/or sheet-shaped crystalline materials of metallic methyl acetylide; conducting first thermal treatment for the rod-shaped crystalline materials and/or sheet-shaped crystalline materials to segregate metal of the metallic methyl acetylide and carbon of the rod-shaped crystalline materials and/or sheet-shaped crystalline materials so as to form carbon nanostructure intermediate which is configured such that carbon containing rod-shaped materials and/or sheet-shaped materials are bound three-dimensionally and then form metal encapsulated carbon nanostructure which encapsulates the metal in the carbon nanostructure intermediate; contacting the metal encapsulated carbon nanostructure with nitric acid; and conducting second thermal treatment for the metal encapsulated carbon nanostructure to blow off metal encapsulated in the metal encapsulated carbon nanostructure.